1. Field of the Invention
The invention relates to an electric power steering system.
2. Discussion of Background
Conventionally, there has been known an electric power steering system (hereinafter referred to as an “EPS”) that assists a driver's steering operation by applying power output from an electric motor to a steering mechanism of a vehicle. For example, a controller of an EPS described in Japanese Patent Application Publication No. 2006-175940 (JP 2006-175940 A) controls an electric motor based on steering torque, a steering angle, and a wheel steered angle obtained from various sensors.
The controller includes first and second reference models (models in which control purposes are formulated). The first reference model defines a relationship between a steering angle and target steering torque, and the second reference model defines a relationship between steering torque and a target steered angle. The controller executes proportional-integral-derivative (PID) control, which is a kind of feedback control, based on the target steering torque and the target steered angle defined by the first and second reference models.
The controller obtains a deviation of actual steering torque from the target steering torque defined by the first reference model, and a deviation of an actual steered angle from the target steered angle defined by the second reference model, and controls the electric motor so as to eliminate the deviations. Through the control, the controller causes the actual steering torque to follow the target steering torque, and causes the actual steered angle to follow the target steered angle.
In the EPS described in JP 2006-175940 A, a rack-and-pinion mechanism is employed as a steering mechanism. The mechanism changes the orientation of steered wheels by converting rotation of a pinion, which is generated in response to a steering operation, into a linear motion of a rack shaft that meshes with the pinion. The rack shaft is slidably housed in a housing. Usually, when the rack shaft reaches a limit of a movable range, so-called “end contact” occurs, that is, an end of the rack shaft (a rack end) contacts the housing, so that a range of movement of the rack shaft is restricted physically.
There may be a situation where steering torque is further applied through a driver's steering operation although the end of the rack shaft is in contact with the housing. This situation may occur, for example, when the vehicle is being parked in a garage. The foregoing second reference model defines the target steered angle that is an ideal steered angle corresponding to steering torque. Therefore, the target steered angle defined by the second reference model increases with an increase in steering torque, but the actual steered angle does not increase. In other words, even if the feedback control of a steered angle is executed, the deviation of the actual steered angle from the target steered angle is not eliminated and therefore the feedback control does not work. In such a case, the controller controls the electric motor so as to eliminate such a deviation, and applies excess assist torque in a steering direction.
When the above-described situation is changed to a situation where the actual steered angle is allowed to follow the target steered angle because, for example, a steering wheel is operated in a direction opposite to the direction in which the steering wheel has been operated, the assist torque is corrected such that the direction and magnitude of the assist torque become appropriate. However, a change in the assist torque during the course of correction may give a driver a sense of discomfort.